Research on biosensor technology has, during the last two decades, been very intensive, as shown by the numerous publications and patent applications submitted worldwide. See, e.g., Janata, J., Jasowicz, M., and DeVaney, D. M., (1994) Anal. Chem. 66:207R-228R. The reasons for these efforts are the potential advantages that such techniques offer compared to alternative, already established, analytical procedures, such as high performance liquid chromatography, bioanalytical procedures such as radio- or enzyme labeled immunoassays, and spectrophotometric bioassays. The advantages of biosensor technology include simplicity and rapidity, reduction of the need for sample pre-treatment, the low cost of the analytical equipment, avoidance of the use of hazardous chemicals, the wide range of analytes which may be measured, and excellent selectivity and sensitivity. However, affinity biosensors have shown generally poor results, since the operational principle involves only a binding reaction between the analyte and the recognition element in the biosensor. Thus, no analyte conversion occurs. This makes it necessary either to use various labeling techniques or to use mass-change sensitive transducers such as potentiometric, optical and piezoelectric transducers. Turner, A. P. F. (1994) Curr. Opin. Biotechnol. 5:49-53. Instruments of this type are often very sensitive to non-specific interactions with the matrix.
Magnetism has been used for specific isolation on an industrial scale. An example is the so called HGMS-technique (High Gradient Magnetic Separator), which is used in large-scale processes such as water purification, treatment of kaolin, and cell/protein isolation with magnetic particles. Also available commercially are inductive sensors that are based on the principle that a coil measures a change in the magnetic field. These inductive sensors react on physical means such that, for example, a magnet or a ferromagnetic material is connected to a float in a tank, with the inductive sensor responding to the level of the solution in the tank.
Several current techniques include measurements of magnetic fields or magnetic permeability for the determination of the content of magnetic material in a sample. Examples of magnetic materials whose presence in samples may be thus detected are: magnetite in ironstone, iron oxide in slag, catalytically active nickel in the process for hydrogenation of fat, iron in oils, and ferromagnetic materials in cement mixtures. Measurements of magnetic fields or magnetic permeability are also used in detection of paramagnetic ions, control of hardmetal alloys, and in magnetic oxygen analyzers. These determinations are not based on molecular recognition, and therefore they often provide a low specificity. These methods are also limited by the fact that they can detect and/or quantify only ferromagnetic materials, and are not adapted to detection of nonmagnetic materials.
Magnetically active reagent carriers as disclosed in U.S. Pat. No. 5,200,270 are useful for the transportation of antibodies, enzymes and haptens in, for example, immunoassays. The '270 patent only describes the use of the carrier for isolation or cleanup, while it is suggested that this can be combined with other chemical analysis techniques. The measurement of magnetic permeability on the carrier particles themselves is not mentioned. The same is true of Japanese patent application JP 62118255 A, which discloses the use of magnetic particles in a similar way for chemical analyses, where the measurement is based on light scatter phenomena.
Measurement of changes in a magnetic field have been reported, such as in combination with oxygen-consuming magnetotactic bacteria, for prediction of aquatic hypoxia, as disclosed in U.S. Pat. No. 5,270,644. This patent describes measurements of magnetic fields on layers of bacteria containing magnetic particles. The abundance of these bacteria in such layers contributes to the strength of the magnetic field, and correlates negatively with the oxygen content in the surrounding environment.
Biomagnetic neurosensors have also been reported. Babb, C. W., Coon, D. R., and Rechnitz, G. A., (1995) Anal. Chem. 67:763-769. This work is based on measurements of magnetic fields that arise from movements of ions in a nerve. Such movements create an electrical current and give rise to an induced magnetic field. This type of magnetic measurement, however, is not based on measurement of a material constant. These kinds of measurements of magnetic fields may be distorted by the great amount of electronic noise in many ambient settings. Furthermore, only substances which can affect the bacteria or nerve, in these examples, can be detected, and the complex response can be very difficult to interpret.
Relative magnetic permeability, .mu..sub.r, is a material constant. This material constant constitutes a measure of a substance's ability to contain and contribute to an externally applied magnetic field. As examples of substances that are classed as ferromagnetic materials (.mu..sub.r &gt;&gt;1), the transition elements and phases of the following elements are included: iron, nickel, cobalt, gadolinium and manganese, as well as chemical compounds, semiconductors, or alloys containing these elements. The relative magnetic permeability for practically all other materials is about 1. See Table 1 and Djurle, E., (1983) Electricitetslara, Teknisk Hogskolelitteratur i Stockholm AB, Sweden. The relationship between magnetic permeability (.mu.) and relative magnetic permeability (.mu..sub.r) can be described according to the following formula: EQU .mu.=.mu..sub.r .times..mu..sub.0, where .mu..sub.0 is a constant with a value of 4.pi..times.10.sup.-7 H/m.
TABLE 1 ______________________________________ Substance Material constant .mu..sub.r ______________________________________ Lead 0.999983 Copper 0.999990 Water 0.999910 Platinum 1.000293 Aluminum 1.000021 Air 1.00000036 Iron 600-1,000,000 ______________________________________
The specific chemical detection or quantification of a substance demands the presence of a recognition element which recognizes the analyte. Kriz, D., (1994) Towards chemical sensors, Lund, Sweden. Molecular recognition between the recognition element and the analyte can be based on many different types of interactions, such as electrostatic, hydrophobic, hydrogen bonds and van der Waals forces. After the initial recognition step, the chemical event is transformed to a physically measurable signal based on, for example, electrochemical, mass, optical, and/or thermometrical properties. In contrast, the present invention is based on changes in magnetic permeability for detection or quantification of a substance. These changes can be correlated to the amount of analyte in the measurement solution, providing a new and advantageous mode of chemical detection and quantification.